Move bubbles away from the IME if needed.

This involves adding the PinnedStackListenerForwarder, so that sysui can have multiple pinned stack listeners listening for updates from the WM. This looked easier and simpler than modifying all the WM code to support multiple listeners. We're also planning to integrate PIP and bubbles at some point, so that they're aware of each other and move together. At that time, we can simply delete the forwarder and use a single listener again, without modifying WM code.

Test: atest SystemUITests
Change-Id: Ie2f9f937fe0a19cac5a1ae83d83698db8d53aba2